1. Field of the Invention
This invention relates to a fuel assembly design for use in a nuclear reactor and particularly to a fast breeder reactor utilizing plutonium as a fuel and pressurized light or heavy water as a reactor coolant and moderator.
2. Description of the Prior Art
The advantages of utilizing nuclear breeder reactors which convert fertile material into fissile material and generate heat, e.g. for power generation, have been widely recognized in view of the limited known fissionable material resources of the world. Development of breeder reactors which convert the more abundant fertile uranium-238 into fissile plutonium-239 utilizing the latter as a fuel, possibly in conjunction with plutonium generated in other known reactors, and breed more fissionable material than is consumed, is highly desirable. Since extensive technological development and experience exists in the design and construction of pressurized light and heavy water reactor plants, use of the pressurized water technology in a breeder application represents an attractive alternative to development of other breeder options.
Heavy water, deuterium oxide (D.sub.2 O), has essentially the same physical and chemical properties as light water, H.sub.2 O. Its nuclear properties, however, are different, the neutron absorption cross section and slowing down power of D.sub.2 O being markedly lower than that of H.sub.2 O. Hence, the use of D.sub.2 O as a coolant in a fast breeder application is desirable due to its nuclear characteristics and the applicability of pressurized water technology. In a plutonium-uranium-deuterium oxide (Pu--U--D.sub.2 O) reactor system, as the coolant to fuel atom ratio decreases, it is known that the conversion or breeding ratios increase. The breeding ratio is the ratio of the number of fissile atoms produced to those consumed. High breeding ratios, approaching a value of 1.40, may be realized in a Pu--U--D.sub.2 O system if a fuel lattice geometry is developed wherein moderator to fuel volume ratios are adjusted to yield moderator to fuel atom ratios approaching 1.0 or less. As the selection of a moderator to fuel atom ratio defines the volume of coolant per unit mass of fuel, it can be appreciated that difficulties arise in designing a fuel lattice capable of passing adequate cooling flow rate at low moderator to fuel ratios. The high flow rates needed to assure adequate reactor core cooling necessitate high velocities in flow channels that are significantly restricted when achieving a low moderator to fuel ratio. In the tightly packed fuel pin lattices, the use of conventional spacer grids is disadvantageous owing to inherent limits in fuel pin packing due to the interposed grids, a tendency to flow induced spacer grid vibration, the parasitic absorption of the grid plate material, and the increase in hydraulic pressure loss resulting from introduction of grids within the restricted flow passages.
The prior art teaches heavy water moderated and cooled reactor designs for particular fuel "rod" diameters and spacings within a moderator to fuel atom ratio range from 0.35 to 4.0 and suggests that a moderator to fuel atom ratio of approximately 0.3 can be achieved in a fuel lattice utilizing touching fuel rods arranged in a triangular pitch. Reduction of heat flux to the degree necessary to avoid potentially destructive hot spots at fuel pin contact points, however, would severely limit the capability of operating such a core at pressurized water reactor conditions. Furthermore, close spacing of the fuel pins may lead to plugging by solid particles carried by the coolant and prohibitively high reactor coolant pumping power requirements. Other difficulties become readily apparent. On the one hand, elimination of spacer grids is desirable in order to permit the higher coolant flow velocities needed to approach the moderator to fuel atom ratios yielding the high conversion ratio of the touching fuel rod configuration. On the other hand, elimination of spacer grids may result in imprecise fuel pin spacing, flow induced vibration and unequal cooling.